On the basis of the above, it can be noted that telecommunication systems may have situations in which there are several parallel or competing communications to one subscriber. These communications can be, for example, group calls. The situation may be such that the subscriber is connected to a so-called basic communication group, which is organization-specific and by means of which information on the daily routines is conveyed within the network. During special incidents, such as emergencies, it may be necessary to create inter-organization communication groups to which some of the subscribers of the different organizations must immediately be connected. In such a case, it is necessary to create communication groups into which subscribers can be forced in order to make them participate even if they were currently taking part in some other communication, for example a previous group call. It may also be necessary to make subscribers stay in a communication created for a special incident even if another communication, such as a new call or group call, were to begin in the middle of the communication in question.
Need to direct subscribers into a priority communication arises especially in case a large part of the communication within the system is group communication, as is the case in the PMR systems mentioned above, and the subscriber may participate in several group communications, or if the subscriber receives several individual communications, or if individual communications are competing with one or several group communications. In such a system there might be need to provide one or more of the communications with a special status so that primarily traffic of the communications that have the special status is forwarded by the subscriber station to the user. Giving the special status is carried out by providing the calls with different priority levels.
In a prior art solution, disclosed in the U.S. Pat. No. 5,257,416, each communication is allocated a dedicated channel which a mobile station checks in order to find out whether there is communication traffic for the station on that channel. This means that the subscriber station scans different radio channels and if there is traffic on several channels the station checks that it is on the correct channel in accordance with the priority hierarchy.
Giving priority to the communications received by a subscriber or subscribers represents the prior art solution to the problem. The prior art solution, however, is not applicable to a situation in which priority tables stored for example in the subscriber stations of the subscriber are continuously changed. The previous solution causes problems because all the communication identifications are within the same area, which means that when adding identifications it is difficult to keep the priority tables in the desired order. As new numbers are added, two kinds of priority table assignments can be employed:
In an absolute priority table assignment, an absolute position is given, for example the 1st, 3rd or 5th. A problem with this is that in order for the assignment to be clear, the position must be unoccupied and it must be known accurately. In addition, to keep the implementation simple, the position should be the same for all the subscribers for whom the new identification is set. Such requirements are very restrictive if identifications are to be added and deleted often, and particularly if common communication identifications between organizations are to be employed.
In a relative priority table assignment, a relative position is given, for example the highest, lowest or in the middle. Such assignments are very imprecise particularly in situations in which the settings are frequently changed, resulting in that the outcome often differs from the desired one. Particularly the communication identifications in the middle are difficult to organize in a precise order.
Thus, it has become evident that when following the prior art solutions it is difficult to implement a method and an arrangement wherein subscribers could be directed to a desired communication when they are already engaged in a communication. It is especially difficult to divert subscribers to a new communication and yet provide them with an opportunity to flexibly return to the previous communication.
It must in addition be noted that to set a subscriber in a force-controlled mode and to reinstate the preceding mode, as in the prior art solutions, takes up a lot of capacity as this has required storing the previous data, forwarding the new data and re-transmitting the previous data in case the data has been stored at the exchange and updated via it.
It must furthermore be noted that a drawback of the prior art solutions is that the former systems do not support continuous dynamic change of a subscriber's communication mode in a situation where the subscriber is shifting between a selected and a fully controlled communication mode.
In prior art solutions, a drawback is that subscribers cannot be forced in a communication without having to interfere with that communication which the subscriber abandons, which results in that the previous communication has been entirely or partly released. This creates a serious problem, because a group call in particular cannot always be released without removing also such subscribers off the communication that were intended to be kept in the existing communication. It must furthermore be noted that a temporary release of a group call may lead into the resources allocated to that call being handed over to another call after the release. Naturally, that is undesirable and unnecessarily hinders communication.